In the processing of plastics via plastic melts, the plastic is typically colored by masterbatches. Masterbatches are plastic granulates having a high content of coloring agent, which are added to the plastic during the processing via the melt in order to color the plastic. The use of masterbatches has the disadvantage that a separate masterbatch must be manufactured for every type of plastic and every color and must be stored ready for the plastic processing. Rapid changes to new colors or changes of the color tone are not possible, since a new masterbatch must be produced in each case.
Instead of masterbatches, liquid colorants may also be used for coloring plastics, the liquid colorant either being supplied with the noncolored plastic granulate to the machine which processes the plastic or being mixed with the already melted plastic in the machine. In this method, a liquid colorant mixed for the desired color tone is used, so that a corresponding liquid colorant must be produced and stored for every desired color. Rapid changes to new colors or a change of the color tone are also not possible here, since a new liquid colorant must be manufactured in each case.
Supplying multiple liquid colorants separately to a plastic processing machine and manufacturing plastics having different color tones via recipe-controlled dosing of the quantities of the individual liquid colorants has been suggested in WO 02/087849. This method has the disadvantage, however, that in the event of recipes having greatly differing mixing ratios of the individual liquid colorants, the mixing of the liquid colorants with the plastic is not sufficiently homogeneous on many plastic processing machines, so that unevenly colored products are obtained.
Therefore, there is a need for a mixer and a mixing method, by which homogeneously mixed liquid colorants for coloring plastics may be manufactured from individual liquid colorants according to a recipe, even with greatly differing proportions of the individual liquid colorants, the manufacture of the mixture being performed directly before the processing of the plastic in accordance with the demand of the plastic processing machine. The mixer must be capable of reliably mixing even thixotropic liquid colorants, which only flow at a specific shear load.
WO 99/34905 discloses a mixer for manufacturing paints, in which individual liquid colorants and one or more lacquer base materials are dosed into a shared chamber of the mixer and mixed in a downstream turbine mixer, before the mixture leaves the mixer. However, this mixer is less suitable for mixing liquid colorants of differing viscosity and density without adding larger quantities of a lacquer base material, as is necessary for coloring plastics, since the mixing of the liquid colorants then remains incomplete.
U.S. Pat. No. 3,664,638 discloses static mixers for mixing liquids, in which multiple helical mixing elements, each having an alternating rotational direction, are positioned in a pipe one after another. Static mixers of this construction are known to those skilled in the art under the names helical mixers or Kenics mixers and are typically used for mixing reactive components during the manufacture of two-component plastics or two-component adhesives.
Dynamic mixers, in which the mixing elements known from the helical mixers rotate in a mixing pipe, are also used for manufacturing two-component plastics. In these mixers, the two components are axially supplied to the mixing pipe at one end as in the static mixers.
The known static helical mixers and the mixers having rotating helical mixing elements using axial supply of the components are not suitable for mixing liquid colorants having different flow behaviors, however, as are used for coloring plastics, in the event of greatly differing mixing conditions. In the event of different viscosities of the liquid colorants to be mixed, there is uneven mixing of the more viscous components, however, so that mixtures having varying color tone are obtained. Similar problems also arise if the liquid colorants have different densities or one or more of the liquid colorants has thixotropic flow behavior.
JP-A 3-60727 discloses a static mixer in the construction of helical mixers, in which nozzles are positioned in the wall of the mixing pipe in order to mix a fluid with a fluid flowing through the pipe. The mixer is reported to have a high mixing action if small quantities of a fluid are mixed, via the nozzles in the wall of the mixing pipe, into a large fluid stream which flows through the mixing pipe.
EP-A 0 090 257 discloses a mixer for multiple component plastics, which has a mixing chamber having a piston rotating in the mixing chamber, a shear gap forming between the piston and the mixing chamber. The individual components are dosed radially into the shear gap and leave the mixer via an outlet opening opposite the piston. The mixer causes laminar flow in the shear gap and has no devices which move the content of the shear gap in the direction opposite to the outlet opening.